SE508442C2 - Electrodynamic magnetic bearing - Google Patents

Abstract

Device for magnetic suspension of a rotor (10, 20, 30) in relative rotational movement along a predetermined rotation symmetric path with respect to a stator (14, 24, 26, 34, 36). Rotor (10, 20, 30) comprises a rotational body (11, 21, 31) which essentially consists of an electrically conductive unmagnetic material. Fitted at the stator (14, 24, 26, 34, 36) is at least one magnetic member (12, 22, 25, 32, 35), which comprises at least one rotation symmetric magnet (12, 22, 25, 32, 35) concentric with the rotational axis. The magnetic member is disposed to give rise to a rotation symmetric magnetic field concentric with the rotational axis. When and only when the rotational body (11, 21, 31) tends to leave the predetermined path and rotate eccentric, the magnetic field generates induced eddy currents in the rotational body (11, 21, 31), the eddy currents gives rise to restoring stabilising forces.

Description

508 442 It was not until the discovery of "the null flux scheme" that the losses could be reduced and this storage method was taken seriously. The theory behind "the null flux scheme" is carefully described in e.g. "Magnetic suspension ...", Journal of Applied Physics, Vol. 43, No. 6, June 1972 by P.L. Richards and M.

Tinkham, and is based on the fact that unnecessary heat generation due to resistive losses can be compensated away with the help of two motorized magnets, which are used to create an area with a weak magnetic field, in which the electrically conductive material is caused to float.

Edge current bearings according to the state of the art are all based on "the null flux scheme" and are available in several designs. Most are in the form of linear bearings intended for use in high-speed trains, such as the patents US 3,951, 075 and US 3,903,809 in the names Miericke et al, and the Swedish patent specification SE 500 120 in the name Lembke. Lembke also proposes the use of the bearing for rotating shafts, which is also mentioned in patents US 3,779,618 in the names Soglia et al and US 3,811,740 in the names Sacerdoti et al.

These bearings according to the state of the art all further use a method to reduce the losses, namely to let all static loads such as dead weight etc. be carried by a separate magnetic relief device in the form of e.g. an attractive permanent magnet. No extra eddy currents are then needed to give rise to this force, but the rotor can center in the middle between the magnets, where the losses are lowest.

Although the losses have been significantly reduced by both the "null flux scheme" and said relief device, a disadvantage of magnetic bearings according to the prior art is that the losses are still too high to enable a commercial application of the bearings. Although Lembke's proposed bearings in experimental experiments have proved to be relatively good, especially with regard to axial bearing, radial bearings of this type still cause problems. Among other things, the thermal expansion of the rotor has been shown to affect the losses considerably, as the rotor can no longer center exactly between the magnets.

According to Richards, "the null flux scheme" offers the possibility of obtaining infinitely low losses, provided that the electrically conductive material, in his case the rails, is infinitely thin and that the magnets are infinitely strong. Furthermore, the speed must be infinitely high. The practical limitations are obvious, and although the best magnets available today have been used in conjunction with very thin discs, no really satisfactory results have been achieved.

The basic reason for the disadvantages of magnetic bearings according to the state of the art is that the area where the field has "zero flux", ie lacks a normal component, is infinitely thin in itself, so only an infinitely thin disk can be used.

If the disc has a thickness, the surface layers will be exposed to an alternating magnetic field when the disc passes the magnets, whereby unnecessary eddy currents occur.

Disclosure of the Invention The object of the present invention is to provide a device for storing elements in relative rotation which completely eliminates unnecessary losses caused by induced eddy currents. Another object of the present invention is that the device should be insensitive to thermal expansion.

The above objects are achieved by a device which exhibits the features described in the claims. The device operates according to the electrodynamic repulsion principle, where a rotor of an electrically conductive material rotates relative to a stator, which comprises magnets, which give rise to a rotationally symmetrical magnetic field which is concentric with the axis of rotation of the rotor. A magnetic field in this way has the property that it does not appear to have an alternating field component for any volume element passing on the rotor.

Since the normal component of such a field does not generate any eddy currents, it thus does not need to be zero, in contrast to what applies to "the null flux scheme". Consequently, the electrically conductive material need not be infinitely thin, nor does it need to be precisely positioned relative to the magnets. The bearing is thus also not sensitive to thermal expansion.

Description of the invention The invention is further elucidated by describing exemplary embodiments based on drawings, in which: FIGURE 1 shows a schematic sketch of a part of an embodiment according to the present invention, where a simple magnet is used; FIGURE 2 shows a schematic sketch of a portion of another embodiment of the present invention, using two magnets; FIGURES 3a and 3b show possible magnetization directions for ring magnets; FIGURE 4 shows a schematic sketch of the ring magnets in a preferred embodiment according to the present invention, where sets of axially stacked axially magnetically oriented ring magnets are used; FIGURE 5 shows a schematic sketch of a part of a further embodiment according to the present invention, in which a shaft is mounted at both its ends; FIGURE 6 shows a sectional view through the bearing in the embodiment shown in Figure 2, where an eccentricity occurs in the moving part; FIGURE 7 shows a sectional view through the bearing in the embodiment shown in Figure 2 without eccentricity; FIGURES Ba to 8f show in cross section a number of embodiments of magnetic bearings according to the present invention; and FIGURE 9 shows a sectional view of a possible starting bearing.

Illustrative Embodiments In the following, a number of embodiments of the present invention will be described. However, it will be apparent to those skilled in the art that these embodiments are merely exemplary and are not to be construed as limiting the scope of the present claims.

In the following description, the terms rotor and stator are used for parts of the embodiments. Those skilled in the art will of course realize that only the relative motion is important, which is why a stationary "rotor" and a rotating "stator" are also conceivable in principle.

Figure 1 shows an embodiment of the present invention, in which certain parts are removed to expose the essential parts of the embodiment. A rotor 10 comprising an electrically conductive non-magnetic rotating body 11 is rotatable within an annular magnet 12. The rotating body 11 need not be rotationally symmetrical per se, but is preferably carefully balanced with respect to a rotation about a fictitious axis of rotation 13. The annular magnet 12 is formed to give rise to a rotationally homogeneous magnetic field. By rotationally homogeneous is meant 508 442 such a field which in the present rotationally symmetrical embodiment does not appear to have any alternating field component for a volume element passing arbitrarily on the rotating body 11, when the axis of symmetry of the magnetic field coincides with the rotation axis of the rotating body 13.

The ring magnet 12 can be permanent magnets, magnets of electromagnetic nature, as well as superconductors, or a combination thereof. The ring magnet is attached to a stator 14, which in Figure 1 is partially broken away. The rotor 10 can be mounted at its non-shown end with an arbitrary bearing.

With the embodiment described above, eddy currents will not occur in the rotating body as long as it rotates concentrically in relation to the magnets, since the normal component of such a field does not generate any eddy currents.

This applies even if the rotating body has a spread in the radial direction, ie has a certain thickness, or if it is subjected to thermal expansion and thus does not run in the middle of the air gap.

On the other hand, stabilizing currents occur as soon as the rotor is displaced from the center position and begins to rotate eccentrically.

If the bearing is subjected to a disturbance so that it begins to rotate eccentrically, the above-mentioned volume element perceives an alternating magnetic field and centering eddy currents appear in the rotating body 11, which return the rotor to its original position. This is possible because the field has a gradient, ie decreasing in radial direction, seen from the center of the magnet.

Figure 2 shows another embodiment of the present invention. This embodiment comprises, in a similar manner as in the previous embodiment, a rotor 20 which comprises an electrically conductive non-magnetic rotating body 21, which is rotatable inside a first ring magnet 22 and which has a fictitious axis of rotation 23. In this embodiment the rotor is tubular and encloses radially a stationary shaft 26. A second ring magnet 25 is arranged at the stationary shaft 26, which in turn forms part of a stator 24. The ring magnets 22, 25 are designed 508 442 7 in the same manner as described above. The rotor 20 can be mounted at its end (not shown) with an arbitrary bearing.

A ring magnet can have its magnetic dipole directed in two fundamentally different directions. These two pure cases are sketched in Figures 3a and 3b, where arrows indicate the direction of the magnetic dipole. In the magnet of Figure 3a, the magnetic dipole is directed parallel to the axis of rotation of the ring magnet, which magnet is referred to as axially magnetically oriented, while the magnet of Figure 3b has the magnetic dipole directed perpendicular to the axis of rotation of the ring magnet, being referred to as radially magnetic oriented.

By placing two axially magnetically oriented ring magnets 22, 25 concentrically in the same plane, as in Figure 2, with the dipoles facing in the same directions, one can amplify the magnetic gradient formed in the gap between the ring magnets 22, 25. In this way, two radially magnetically oriented ring magnets 22, 25 can be placed concentrically in the same plane, with the dipoles facing in the opposite direction, and thus give rise to an amplified magnetic gradient. Since the rotor 20 rotates in the gap between the magnets, the gradient of the radial magnetic field component perceived by the rotating body 21 is increased (while the tangential remains zero), provided that the axis of rotation 23 of the rotor coincides with the axis of symmetry of the magnets. In the event of a disturbance, ie displacement of the axis of rotation 23 of the rotating body, the restoring force formed by the induced eddy currents will be stronger than in the case of a single ring magnet. A stiffer suspension is thus obtained.

Another way to amplify the restoring force is to create a large radial magnetic field component along a larger distance along the rotating body. This can be done by, instead of placing additional ring magnets radially relative to the first, placing them axially relative to the first. To maximize the size of the radial gradient 508 442 8, the ring magnets should be placed with the polarity reversed. This applies to both axially magnetically oriented and radially magnetically oriented ring magnets.

A preferred embodiment of course consists of a combination of the two methods described above for amplifying the magnetic effect. The configuration of the magnets in such an embodiment, with axially magnetically oriented magnets, is shown in Figure 4. By having two concentrically arranged series of alternately directed ring magnets, which give rise to an intermediate gap, in which the rotor can rotate, it can the magnetic effect is multiplied.

Figure 5 shows a special embodiment of the present invention. A completely tubular shaft 31 in this case constitutes a body of rotation and has magnetic bearings with ring magnets 32, 35 arranged at each end. The shaft 31 is electrically conductive and simultaneously serves both as a shaft and as a bearing. The ring magnets 32, 35 are attached to a stator 34 and a shaft 36 arranged therefor. The construction can thus be made very light, and at the same time flexurally rigid, which means that it can be used at very high speeds.

Figures 6 and 7 show how the restoring forces act in a device according to the present invention. Figures 6 and 7 show a bearing corresponding to the embodiment shown in Figure 2 where the stator and the stationary shaft have been omitted, but the principles are the same also for other possible embodiments. In Figures 6 and 7, most of the designations are the same as in Figure 2. In Figure 6, the axis of rotation of the rotating body 21 is displaced relative to the axis of symmetry of the magnetic field generated by the ring magnets 22, 25. A volume element 28 on the rotating body 21 will experience a varying magnetic field along its direction of movement, whereby an eddy current arises in the volume element 28. This eddy current will give rise to a force which counteracts the movement. The total force resultant acting on all the volume elements in the rotating body will be directed upwards in the figure and is denoted by F.

In Figure 7, the rotating body 21 is concentric with the magnetic field and no eddy currents occur in the rotating body 21, and thus the total force resultant on the rotating body 21 is zero.

Different applications of magnetic bearings according to the present invention entail different preferred embodiments. Figures 8a to 8g illustrate some interesting embodiments in sectional views. Figure 8a shows the corresponding embodiment as in Figure 2, with two concentric ring magnets, one inside and one outside a rotating cylindrical shaft. Figure 8b shows an application where a rotor with conical portions at the storage positions is used. Such an embodiment gives a small axial bearing effect even in the case of a non-vertical placement of the axis of rotation. Fig. 8c shows an embodiment similar to that shown in Fig. 4, but with three pairs of ring magnets. The embodiment in Figure 8d also has three pairs of annular magnets, but between these there are in this embodiment annular soft iron discs 49. This arrangement concentrates the magnetic flux to the area between concentric iron discs, whereby the magnetic force at these positions is amplified. Fig. 8e shows an embodiment in which three axially stacked ring magnets with intermediate iron plates act as bearings. In this embodiment, however, the rotor comprises two concentric pipe sections, which move outside and inside the ring magnets, respectively. The magnetic field is then used to influence the axis both externally and internally about the magnets. Figure 8f shows an extension of these thought paths, in which two concentric sets of ring magnets are used together with a rotor comprising three concentric tubular parts. Figure 8g shows an embodiment which, at the expense of some of the efficiency of the radial bearing, obtains a light axially acting bearing. In this embodiment, a rotor is surrounded by three outer and three inner ring magnets, as in Figure 8c. However, in this embodiment the rotor inside 508 442 also also comprises two iron rings 48 which are placed along the axis of rotation in line with the spaces between the three pairs of ring magnet. These iron rings 48 will reduce the radial bearing, compared to the embodiment of Figure 8c, but with an axial displacement of the rotor, the change in the magnetic flux will tend to return the rotor to its original position. The iron rings 48 can either be placed inside the rotor or outside it.

The ring magnets mentioned above can of course be replaced by magnets with other rotationally symmetrical geometries.

The magnets can consist of various different types of magnets, or combinations thereof. A permanent magnet is a simple solution at high speeds, where the high speeds give rise to strong repulsive forces. The permanent magnets work worse at low speeds or at standstill. For the same reason, electromagnets fed with direct current are excellent at high speeds, while at low speeds high currents are required to give rise to sufficiently strong return forces.

Superconducting magnets can be used to advantage. A solution at low speeds is to use electromagnets supplied with alternating current, which can even handle a floating contactless storage for a stationary rotor. However, AC-supplied electromagnets are less stable at high speeds. A preferred embodiment comprises the combination of a permanent magnet and an alternating current-supplied electromagnet, whereby a stable storage for all speeds can easily be obtained.

The electromagnets have the advantage that you can vary their strength during operation and thereby adapt the properties of the magnetic bearing. The stiffness of the bearing, ie. with how much force a movement from the ideal path is prevented, can be easily set, e.g. depending on the speed at which the rotor rotates. When using a combination of static and fluctuating magnetic fields, it is advantageous to change the mutual strength relationship between these two types. When accelerating a rotor from 508 442 ll stationary to a high speed, it is advantageous if the gear field dominates from the beginning, at the low speeds, after which the static field as the rotor accelerates takes over. This can be realized by controlling the currents and / or the frequencies of the currents transmitted through the electromagnets.

As the bearing properties are not fully developed at low speeds, these can either be improved by making the tubular shaft thicker-walled or supplemented with some form of starting bearing.

The simplest form of starting layer is to coat magnets and possibly. shaft with a thin material with good sliding properties, e.g. teflon. At start-up, the shaft slides on the sliding surface until the speed is so high that the shaft lifts and is stabilized by the magnetic field.

Instead of plain bearings, ball bearings can of course be used, which are then given a diameter which is slightly larger than the shaft in question.

The method is common as so-called emergency bearings to active magnetic bearings.

Air storage is a better method. In this method, air is pumped during start-up through small holes drilled between the magnets along the length of the shaft, where a supporting air cushion is formed.

The best starting method is to use an alternative magnetic bearing of the simplest kind. This bearing only needs to function axially, but can be designed so that it still provides a passive radial stability. The bearing does not have to be able to function at high speeds, which is why electronics can be made significantly cheaper than for conventional active magnetic bearings.

An example of such a combined bearing is shown in Figure 9. A rotating shaft is provided at both ends with iron fittings 50. These stand in the middle of electromagnets 51 fixed in the stator, which are controlled by a simple control electronics 52. Other reference numerals denote previously described details . The magnets provide radial stability at high speeds and the iron ring provides axial stability at all speeds, but also radial instability, as previously described. The electromagnet stabilizes the bearing radially and enhances the axial stability. At high speeds, the electromagnet with its control system can be disconnected.

If the electromagnet is not switched off at high speeds, it can be used for measuring and / or compensating for mechanical forces on the rotor.

In the above-described embodiments, a few embodiments of the present invention have been described. It will be appreciated that the characterizing features of the present invention may be combined in many different configurations and combinations, all of which are within the scope of the claims.

Claims (10)

508 442 13 PATENT CLAIMS Device for magnetic storage, according to the electrodynamic repulsion principle, of a rotor (10, 20, 30) in relative rotational motion along a predetermined rotationally symmetrical path relative to a stator (14, 24, 26, 34, 36) , which rotor (10, 20, 30) comprises a rotating body (11, 21, 31) which consists essentially of an electrically conductive non-magnetic material and which stator (14, 24, 26, 34, 36) has at least one magnetic member ( 12, 22, 25, 32, 35) arranged separately, characterized in that the magnetic member (12, 22, 25, 32, 35) comprises at least one rotationally symmetrical magnet (12, 22, 25, 32) concentric with the axis of rotation 35) and is arranged to give rise to a rotationally symmetrical magnetic field concentric with the axis of rotation, the magnetic field generating induced eddy currents in the rotating body (11, 21, 31) and thereby returning stabilizing forces only when the rotating body (11, 21, 31) tends to leave the predetermined path and r otera eccentric. Magnetic storage device according to claim 1, characterized in that at least one rotationally symmetrical magnet (12, 22, 32) is arranged around the rotating body (11, 21, 31). Magnetic storage device according to claim 1 or 2, characterized in that at least one rotationally symmetrical magnet (25, 35) is arranged inside the rotating body (11, 21, 31). Magnetic storage device according to claim 1, characterized in that at least one inner rotationally symmetrical magnet (25, 35) is arranged inside the rotating body (11, 21, 31) and that at least one outer rotationally symmetrical magnet (22, 25) is arranged around the rotating body. (11, 21, 31) concentric with and in the same plane as the inner rotationally symmetrical magnet 508 442 14 "(25, 35), the dipoles of the inner and outer magnets being directed in the same directions if the magnets are axially magnetically oriented and directed in opposite directions. direction if the magnets are radially magnetically oriented. Magnetic storage device according to one of the preceding claims, characterized in that at least two rotationally symmetrical magnets (25, 35) are arranged axially and concentrically relative to one another, with their dipoles directed in alternately opposite directions. Magnetic storage device according to one of the preceding claims, characterized in that at least one of the rotationally symmetrical magnets (25, 35) consists of an electromagnet supplied with direct current. Magnetic storage device according to claim 6, characterized in that the electromagnet consists of a superconducting electromagnet. Magnetic storage device according to one of Claims 1 to 5, characterized in that at least one of the rotationally symmetrical magnets (25, 35) consists of an electromagnet supplied with alternating current. Magnetic storage device according to claim 8, characterized in that at least one of the rotationally symmetrical magnets (25, 35) is constituted by a permanent magnet. Magnetic storage device according to claim 9, characterized in that the current and the frequency of the alternating current through the electromagnet can be regulated during operation.